style(frontend): look&feel v2 — layered shadows, sharper radii, mono data register

Colors (GSD palette) untouched. New visual vocabulary across the whole GUI:
- warm-tinted layered shadow tokens (--shadow-xs/sm/md) replacing flat rings;
  cards/dialogs framed with hairline border + real elevation
- strict radii scale: xl cards / lg inputs / md controls / 2xl dialogs;
  buttons move from pills to rounded-md
- third type register: --font-mono + tabular-nums on SQL, timers, dates, ids;
  standardized .thot-label micro-label tier
- prose: framed tables with uppercase headers, elevated code blocks
- SqlViewer badges moved from hardcoded Tailwind greens/reds to success/
  destructive token tints; ResultsPanel inline styles replaced with classes
- artifact gate: first option rendered as filled primary action

Verified offline via Playwright against a mock backend replaying a real psd
session (no VPN needed). tsc clean, 115/115 vitest.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/viewers/MarkdownView.tsx

@@ -16,7 +16,7 @@ function MermaidBlock({ code }: { code: string }) {
       );
   }, [code]);
 
-  if (error) return <pre className="text-red-500 text-xs">{error}</pre>;
+  if (error) return <pre className="text-destructive text-xs">{error}</pre>;
   if (!svg) return <pre className="text-xs opacity-50">{code}</pre>;
   // eslint-disable-next-line react/no-danger
   return <div dangerouslySetInnerHTML={{ __html: svg }} />;
@@ -36,7 +36,7 @@ const components: Components = {
     );
   },
   pre({ children }) {
-    return <pre className="overflow-auto rounded bg-muted p-3 text-sm">{children}</pre>;
+    return <pre className="overflow-auto rounded-lg bg-muted p-3 text-sm shadow-xs">{children}</pre>;
   },
 };